This invention relates to improvements of the inventions disclosed and claimed in my Pat. Nos. 3,810,061, May 7, 1974, for HIGH-VOLTAGE FUSE and 3,848,214, Nov. 12, 1974, for METHOD OF ASSEMBLING ELECTRIC HIGH VOLTAGE FUSES AND SUBASSEMBLY THEREFOR.
The aforementioned patents refer to fuses having plug terminals, i.e. fuses wherein a tubular casing of electric insulating material is closed on both ends by cylindrical terminals inserted or press-fitted into it, and plugging the same. The plug terminals are conductively interconnected by substantially helically wound fusible element means, e.g. one or more ribbons of silver. These fusible element means are not supported by conventional mandrel structures of electric insulating material, e.g. a ceramic material, but their support is, in essence, effected by a pulverulent arc-quenching filler, e.g. quartz sand, forming a dense packing for the fusible element means which are embedded in it.
Manufacturing a fuse in accordance with the aforementioned Pat. 3,848,214 includes the steps of
A. FORMING A SQUIRREL-CAGE-LIKE STRUCTURE BY A PAIR OF CYLINDRICAL COAXIALLY ARRANGED AND FIXEDLY SPACED PLUG TERMINALS AND A PLURALITY OF METAL RODS EXTENDING PARALLEL TO THE JOINT AXIS OF SAID PAIR OF PLUG TERMINALS;
B. WINDING FUSIBLE ELEMENT MEANS SUBSTANTIALLY HELICALLY AROUND SAID PLURALITY OF METAL RODS AND CONDUCTIVELY CONNECTING THE ENDS THEREOF TO SAID PAIR OF PLUG TERMINALS;
C. INSERTING SAID SQUIRREL-CAGE-LIKE STRUCTURE WITH SAID FUSIBLE ELEMENT MEANS THEREON IN COAXIAL RELATION INTO A TUBULAR CASING OF ELECTRIC INSULATING MATERIAL AND AFFIXING SAID CASING TO SAID PAIR OF PLUG TERMINALS;
D. FILLING SAID CASING WITH A PULVERULENT ARC-QUENCHING FILLER THROUGH AN APERTURE IN ONE OF SAID PAIR OF PLUG TERMINALS; AND
E. WITHDRAWING SAID PLURALITY OF METAL RODS FROM SAID FUSIBLE ELEMENT MEANS AND DRAWING SAID METAL RODS OUT OF SAID CASING AT SUCH TIME WHEN SAID ARC-QUENCHING FILLER PROVIDES SUBSTANTIAL SUPPORT FOR SAID FUSIBLE ELEMENT MEANS, THUS ALLOWING TO DISPENSE WITH THE METAL RODS PREVIOUSLY PROVIDING THE NECESSARY SUPPORT FOR THE FUSIBLE ELEMENT MEANS.
The aforementioned metal rods may be removed sequentially or simultaneously after the casing has been filled substantially with pulverulent arc-quenching filler. As an alternative, filling of the casing with pulverulent arc-quenching filler and withdrawal of the supporting rods for the fusible element means may be effected in sequential increments of which each is less than the total length of the internal space inside the casing of the fuse.
It will be understood that whenever a metallic supporting rod for a fusible element is withdrawn from it and from the casing, a void is formed at the points heretofore occupied by the rod. Such voids are almost instantly filled with pulverulent arc-quenching filler which flows into them in liquid-like fashion. In order to achieve complete filling of the casing with pulverulent arc-quenching filler, to avoid any displacement of the fusible element means from their initial position incident to removal of their temporary metallic supporting rods and to minimize the time required to comply with these requirements it is necessary to minimize the voids resulting from the withdrawal of the metallic supporting rods for the fusible element means. This, in turn, makes it necessary to minimize the diameter of the supporting rods for the fusible elements. As a result of a reduction of their diameter, the temporary metallic supporting rods for the fusible element means have a tendency to undergo a deflection or to buckle.
It is the principal object of the present invention to minimize the diameter of the metallic supporting rods for the fusible elememt means and to minimize the deflection to which they are subjected by radial loads and by axial compression.
One means to achieve this end is disclosed in the above Pat. 3,810,061. It consists in an annular brace for the metal rod by which the fusible element means are supported temporarily. Long fuses for high voltage ratings, e.g. voltage ratings in excess of 15 kv, require an arrangement of several annular braces along the metallic supporting rods for their fusible elements. It is, therefore, another object of this invention to provide means which allow to dispense with annular braces as disclosed in Pat. 3,810,061, or to minimize the number thereof.
The aforementioned Pat. 3,848,214 shows supporting rods for the fusible element which are screw-threaded on one end. The screw-threaded ends of the aforementioned rods engage internally screw-threaded bores in one of the plug terminals. If the lengths of the threads on the rods and inside the bores are relatively short, the structure may lack dimensional stability. If the aforementioned length is increased, this results in a thickness of the plug terminals in excess of their functional requirements which consist in carrying current and properly sealing the casing of the fuse.
Affixing the metallic supporting rods for the fusible elements in blind screw-threaded bores in one of the plug terminals results in weakening of the ends of the supporting rods, which may be undesirable.
In order to minimize the diameter of the metallic supporting rods for the fusible elements these rods should be of a material which is as stiff as possible or has a Young's modulus which is as large as possible, e.g. tungstencarbide. Such materials can hardly be machined and thus cannot be provided with the screw-threads called for by Pat. 3,848,214.
The objects underlying the present invention include removal of all of the aforementioned limitations.
The Pats. 3,810,061 and 3,848,214 disclose a modification of the fuse manufacturing process wherein the blind bores in one of the plug terminals and the ends of the supporting rods which project into them need not be screw-threaded. In that particular modification a center post maintains the plug terminals of the fuse during a certain phase of the assembly process in fixed spaced relation, and the metal rods surrounding that center post perform merely the duty of supporting temporarily the fusible element or elements.
The presence of the aforementioned center post makes it more difficult to install a blown fuse indicator in the fuse. It is, therefore, another object of the present invention to provide a fuse assembly method by which the results achieved with the means disclosed in Pats. 3,810,061 and 3,848,214 can be achieved and can be surpassed without resorting to a center post, and which method allows convenient installation of a blown fuse indicator whenever desirable.